Selling property with unpermitted second unit

Let’s say someone has a house in Torrance with a huge amount of sqft and in the back they build a second detached house completely unpermitted. Can the house be sold, and would it bring up or drop the value of the property? I know unpermitted work is not counted as part of the mortgage valuation, but would that house sell for more than one that didn’t have a second unit unpermitted?